Tree Rings Indicate Potential for Severe Soil Moisture Drought in California Is Much Greater Than Observations Suggest

Abstract Tree rings reveal that 1579–1580 was by far the driest pair of years across California since at least CE 1100. Here we produce a gridded tree‐ring reconstruction of summer soil‐moisture anomalies for California and perform a peaks‐over‐threshold extreme value analysis on statewide mean drought severity from CE 1100–2025. Results indicate marginal annual exceedance probabilities (AEP) of ∼0.24% for 1579 and ∼0.04% for 1580, with a two‐year block EP of ∼0.05% for 1579–1580. Considering only the instrumental record (1901–2025), AEPs for the drought severities reconstructed for 1579 and 1580 cannot be robustly estimated and, for 1580, the reconstructed severity appears essentially impossible. Our findings demonstrate that including tree‐ring reconstructions meaningfully changes the expected frequency of droughts on realistic planning horizons, such as a 1% AEP (100‐year) event. Modern risk assessments based solely on observations likely underestimate the true probability of extreme drought events, even without accounting for anthropogenic climate change.